Practical estimation methods of the design loads for primary structural members of tankers

Various design loads used in the strength evaluation of ship structures have been introduced currently by classification societies. As most of these design loads have been determined as the standard loads for the sake of convenience, the relationships between the design loads and the sea states actually encountered by ships seem to be weak. Accordingly, it may be difficult to refer and utilize them as the fundamental design concepts for ship designers or the operational guidelines for ship operators. To overcome these difficulties, the authors have developed practical estimation methods of the design loads having transparent and consistent backgrounds to the actual loads acting on primary structural members of tankers. In this paper, the design sea states that closely resemble the actual sea states which are considered as the most severe for hull structures are firstly proposed. Secondly, the practical estimation methods of the design sea states are proposed by parametric studies using the results of series calculation of representative tankers. Thirdly, the practical estimation methods of design regular waves resulting in the same level of stresses with that induced in irregular waves under the design sea states are proposed. Finally, the practical estimation methods of the design loads such as ship motions, accelerations, hull girder bending moments and hydrodynamic pressures that are induced under design regular waves are proposed.     

Practical estimation method of the design loads for primary structural members of bulk carriers

Various design loads used in the strength evaluation of ship structures have been introduced currently by classification societies. As most of these design loads have been determined as standard loads for the sake of convenience, the relationships between the design loads and the sea states actually encountered by ships seem to be weak. Accordingly, it may be difficult to refer and utilize them as fundamental design concepts for ship designers or as operational guidelines for ship operators. In view of this background, various efforts (Trans. West-Jpn. Soc. Nav. Archi. 89 (1995) 191; 93 (1997) 121; Jap Shipbuild. Mar. Eng. 2 (1967) 13; Proceedings of the 18th International Conference on Offshore Mechanics and Arctic Engineering 1999; Mar. Struct. 13 (2000) 495; J. Ship. Res. 45 (2001) 228; J. Mar. Sci. Technol. 6 (2002) 122; J. Soc. Nav. Archi. Jpn. 186 (1999) 319; Technical guide regarding the strength evaluation of hull structures, 1999; Proceedings of the 14th International Conference on Offshore Mechanics and Arctic Engineering, 1995; Mar. Struct. 10 (1997) 611; Proceedings of the Eighth International Symposium on Practical Design of Ships and Other Floating Structures, Shanghai, Vol. 2, China, 2001, p. 1089; Mar. Struct. 12 (1999) 171; Mar. Struct. 4 (1991) 295) have been made in recent years for developing estimation methods of the design sea states in a rational manner. Furthermore, for more practical purposes, the authors have developed practical estimation methods of the design sea states, the design regular waves and the design loads having transparent and consistent backgrounds to the actual loads acting on primary structural members of tankers (Mar. Struct. 16 (2003) 275).In this paper, it is firstly confirmed that the proposed methods for tanker structures can also be applied to the primary structural members of bulk carriers. Furthermore, the necessary modifications are carried out considering the specific structural types and loading conditions of bulk carriers in order to improve the estimating accuracy of the proposed design loads. Finally, comparisons between the results obtained by the structural analysis of a hold model applying the proposed design loads and long-term values of stresses by the most advanced direct structural analyses for different loading conditions are introduced and discussed in detail.     

Long-term correlation structure of wave loads using simulation

This paper proposes a new method for combining the lifetime wave-induced sectional forces and moments that are acting on the ship structure. The method is based on load simulation and can be used to determine the exceedance probabilities of any linear and nonlinear long-term load combination. It can also be used to determine the long-term correlation structure between these loads in the form of the long-term correlation coefficients. They are essential part of the load combination procedures in design and strength evaluations as well as in the fatigue and reliability analysis of ship structures.The simulation method treats the non-stationary wave elevations during the ship’s entire life (long-term) as a sequence of different stationary Gaussian stochastic processes. It uses the rejection sampling technique for the sea state generation, depending on the ship’s current position and the season. Ship’s operational profile is then determined conditional on the current sea state and the ship’s position along its route. The sampling technique significantly reduces the number of sea state-operational profile combinations required for achieving the convergence of the long-term statistical properties of the loads. This technique can even be used in combination with the existing long-term methods in order to reduce the number of required weightings of the short-term CDFs. The simulation method does, however, rely on the assumption that the ship is a linear system, but no assumptions are needed regarding the short-term CDF of the load peaks.The load time series are simulated from the load spectra in each sea state, taking into account the effects of loading condition, heading, speed, seasonality, voluntary as well as involuntary speed reduction in severe sea states and the short-crested nature of the ocean waves. During the simulation procedure, special care has been given to maintaining the correct phase relation between all the loads. Therefore, time series of various load combinations, including the nonlinear ones, can be obtained and their correlation structure examined. The simulation time can be significantly reduced (to the order of minutes rather than hours and days) by introducing the seasonal variations of the ocean waves into a single voyage simulation. The estimate of the long-term correlation coefficient, obtained by simulating only a single voyage with the correct representation of seasonality, approaches the true correlation coefficient in probability. This method can be applied to any ship and any route, or multiple routes as long as the percentage of the ship’s total lifetime spent in each of them is known.A study has been conducted to investigate the effects of ship type, route and the longitudinal position of the loads on the values of the correlation coefficients between six different sectional loads; vertical, horizontal and twisting moments, as well as shear, horizontal and axial forces. Three ocean-going ship types have been considered; bulk carrier, containership and tanker, all navigating on one of the three busy ship routes; North America-Europe, Asia-North America and Asia-Europe. Finally, the correlation coefficient estimates have been calculated for five different positions along the ship’s length to investigate the longitudinal variation of the correlation coefficient.     

船舶结构疲劳评估设计波法

以散货船船舯内底与底边舱斜板折角处节点为例,对船舶结构疲劳评估的设计波法进行研究.论述了设计波法的基本原理,给出了确定设计波的流程。通过分析该节点的受力情况,确定5个控制载荷参数及设计波参数。以谱分析法得到的参考应力范围为目标值,对5个控制载荷参数对应的设计波作用下的节点应力范围结果进行回归分析,从而得到了用于疲劳评估的设计波组合,并利用该方法对1艘散货船的船舯内底与底边舱斜板折角处的节点进行了计算。结果显示,设计波法与谱分析法的结果吻合较好,计算量大幅减少。     

Hull-girder reliability of new generation oil tankers

A series of new generation oil tankers is presently under construction. These ships differ from traditional oil tankers by their unusual form and therefore direct hydrodynamic analysis is used to determine design vertical wave bending moments instead of adopting IACS rule values. The purpose of the paper is to quantify changes in hull-girder reliability resulting from the new design features. To achieve this, first-order reliability analysis is carried out with respect to ultimate collapse bending moment of the midship cross section of a new generation oil tanker and of a conventional “rule” designed oil tanker. The stochastic model of wave-induced bending moment is derived from direct hydrodynamic analysis performed according to IACS Recommendation No. 34 Standard Wave Data, Rev 1, 2000. The probability distribution of the still water bending moment is assumed based on the data from loading manuals. The model uncertainties of linear wave loads, non-linearity of the response as well as load combination factors are included in the reliability formulation. The reliability analysis is performed for three relevant loading conditions: full load, ballast and partial load and for two states of the hull: the “as-built” hull and “corroded” hull according to anticipated 20-year corrosion. One of the most interesting conclusions from the study is that the annual hull-girder reliability of new generation oil tanker is increased considerably compared to the conventional oil tanker. Sensitivity and parametric studies are performed with respect to random variables representing modelling uncertainties. The results of a sensitivity study enable sorting of pertinent variables according to their relative importance, while parametric study is used to quantify changes in the reliability indices for moderate variation of input parameters. Furthermore, some other results and discussions are presented pointing out the benefits of introducing the ship reliability methods in design practice, especially if this refers to new designs.     

用于轴系校中的船体变形计算研究

船体变形是影响船舶推进轴系校中质量的一个非常重要的动态因素.在综合考虑引起船体变形的三种主要因素的基础上,以76000t成品油轮为例,计算了极限装载状态下的重力分布和极限海况时的波浪载荷.通过对整船有限元模型合理施加重力、浮力及环境温度载荷,并模拟船舶水弹性约束,得到船体二层底的局部变形,为在轴系校中计算中计入船体变形的影响奠定了基础.     

单壳油船改散货船轮机设计中的实施

单壳油船改散货船因其低成本、短工期、船舶运行稳定等特点广受中小航运公司的青睐.特别是前几年散货船市场的空前繁荣,一大批单壳油船被改装成了散货船.就轮机专业而言,因原船配置各异,且在许多方面不能满足现行散货船的相关规范,改装设计过程中需合理设计,以提高其经济性及合理满足相关规范的要求.     

沿海成品油船船型开发调查研究

介绍沿海成品油油运市场对油船的需求情况以及沿海港口对成品油船设计的限制条件,探讨规范的变化对成品油船结构的影响并对现有成品油船技术状况及存在的问题进行分析,总结了现有油船船型的特点,提出开发沿海成品油船新船型的建议。     

Structural loads induced in a containership by abnormal wave conditions

An analysis is presented of the vertical bending moments induced in a containership by a set of abnormal waves measured at different locations and on different occasions. A systematic investigation was carried out by using a large set of wave traces that included abnormal waves. In this way it was possible to assess the influence of the height, length, and shape of the abnormal waves on the wave-induced structural loads. The probability distributions of the ship responses to the sea states that included the abnormal waves were also calculated and were compared to the responses induced by the abnormal waves and to fitted distributions. Finally, the structural loads induced by the abnormal waves were compared with rule values and with long-term predictions.     

Numerical weight function method for the structural analysis of ships: a speedy direct calculation with condensed structural information

The weight function method was originally derived for crack problems to calculate stress intensity factors for arbitrary loading conditions. In this article, a numerical weight function method has been extended to formulate the structural response analyses of two-dimensional elasticity, plate-bending, and three-dimensional plate-structures by using the finite-element method. The solution procedure is based on the well-known Maxwell–Betti reciprocal theorem, which is applied to the original and properly defined auxiliary problems. The present numerical weight function may be considered as a finite-element version of a Green’s function in an integral equation solution scheme. Although ship structures are certainly analysed by the finite-element method in a practical design procedure, the weight function approach has not yet been realized. The method is very useful for the analysis of structures subjected to a vast range of loading conditions, because structural responses can simply be calculated by the inner product of the universal weight function and load vectors. The validity and convergence characteristics of the present method are investigated by two-dimensional elastic and plate-bending problems, respectively. Finally, the method is applied to the calculation of the response amplitude operator of a stress component at a critical structural detail of a double-hull tanker, and the speed and efficiency of the method are quantitatively discussed based on the practical results.     

Comparing different contour methods with response-based methods for extreme ship response analysis

Environmental contours are often applied in probabilistic structural reliability analysis to identify extreme environmental conditions that may give rise to extreme loads and responses. They facilitate approximate long term analysis of critical structural responses in situations where computationally heavy and time-consuming response calculations makes full long-term analysis infeasible. The environmental contour method identifies extreme environmental conditions that are expected to give rise to extreme structural response of marine structures. The extreme responses can then be estimated by performing response calculations for environmental conditions along the contours.Response-based analysis is an alternative, where extreme value analysis is performed on the actual response rather than on the environmental conditions. For complex structures, this is often not practical due to computationally heavy response calculations. However, by establishing statistical emulators of the response, using machine learning techniques, one may obtain long time-series of the structural response and use this to estimate extreme responses.In this paper, various contour methods will be compared to response-based estimation of extreme vertical bending moment for a tanker. A response emulator based on Gaussian processes regression with adaptive sampling has been established based on response calculations from a hydrodynamic model. Long time-series of sea-state parameters such as significant wave height and wave period are used to construct N-year environmental contours and the extreme N-year response is estimated from numerical calculations for identified sea states. At the same time, the response emulator is applied on the time series to provide long time-series of structural response, in this case vertical bending moment of a tanker. Extreme value analysis is then performed directly on the responses to estimate the N-year extreme response. The results from either method will then be compared, and it is possible to evaluate the accuracy of the environmental contour method in estimating the response. Moreover, different contour methods will be compared.     

Evaluation of added resistance in regular incident waves by computational fluid dynamics motion simulation using an overlapping grid system

A computational fluid dynamics simulation method called WISDAM-X was developed to evaluate the added resistance of ships in waves. The Reynolds-averaged Navier–Stokes (RANS) equation was solved by the finite-volume method and a MAC-type solution algorithm. An overlapping grid system was employed to implement rigorous wave generation, the interactions of ships with incident waves, and the resultant ship motions. The motion of the ship is simultaneously solved by combining the solution of the motion of the ship with the solution of the flow about the ship. The free surface is captured by treatment by the density-function method. The accuracy of WISDAM-X is examined by a comparison with experimental data from a container carrier hull form, and shows a fairly good agreement with respect to ship motion and added resistance. Simulations were also conducted for a bow-form series of a medium-speed tanker to examine the effectiveness of the WISDAM-X method as a design tool for a hull form with a smaller resistance in waves. It was confirmed that the WISDAM-X method can evaluate the added resistance with sufficient relative accuracy and can be used as a design tool for ships.     

大型油轮缆力约束的统计分析

建立大型油码头设计船型,明确其各项特征参数,将有助于开展开敞式码头的系泊设计和试验研究。依据Q88油轮数据,采用覆盖率等方法,对10万吨级及以上油轮的缆力约束参数进行了统计分析,确定了大型油码头设计代表船型的系泊缆力约束参数。研究结果对于码头设计和系泊安全研究具有参考价值。     

无压载水舱船型开发和前景分析

阐述了国外的无压载水舱的贯通流船体和V型船体,并以6500t无压载水舱油船为例,提供了主尺度、线型和技术参数。通过与常规油船的分析比较,显示该新船型的优势。为推动无压载水舱的其他船型的开发提供了指导方案。     

A practical method for torsional strength assessment of container ship structures

Container ship structures are characterized by large hatch openings. Due to this structural property, they are subject to large diagonal deformations of hatch openings and warping stresses under complex torsional moments in waves. This necessitates torsional strength assessment of hull girder of container ships in their structural design stage. In this paper, a practical method for torsional strength assessment of container ship structures with transparent and consistent background is discussed based on the results from up-to-date analyses. In order to estimate the torsional response characteristics as accurately as possible, three-dimensional Rankine source method, after being validated by tank tests, is employed for estimation of wave loads on a container ship, and FE analyses are conducted on the entire-ship model under the estimated loads. Then, a dominant regular wave condition under which the torsional response of the container ship becomes maximum is specified. Design loads for torsional strength assessment that give torsional response equivalent to the long-term predicted values of torsional response are investigated based on the torsional moments on several container ships under the specified dominant wave condition. An appropriate combination of stress components to estimate the total hull girder stress is also discussed.     

小型液货船惰性气体系统最新动向

防止油船和化学品船火灾及爆炸事故一直是IMO关注的问题,同时也是油轮公司和化学品船公司日常安全管理的重点之一。液货船(包括油船、化学品船和液化气体船)在海上航行或在货物操作期间,可能发生各种不可预测的因素,存在着很多巨大的火灾或爆炸危险性。为确保安全,避免航行及货物操作期间各种不可预测因素的影响,只有在液货船,如油船上安装惰性气体系统,并对船员作全面培训,才能减少甚至防止事故的发生。     

晃荡对CSR油船设计的影响

为明确船舶营运过程中晃荡载荷对不同船型液货舱构件的影响,基于《散货船和油船结构共同规范》,分析晃荡载荷的形成及决定晃荡载荷的因素,结合纵向和横向晃荡在油船中的具体载荷水平,分析晃荡运动对CSR油船液货舱结构的影响,以VLCC制荡舱壁为优化目标,提出新的设计方案,采用MSC.Patran&Nastran有限元分析软件,论证制荡舱壁局部开孔的可行性。     

波浪引起破损船的垂向运动和弯矩（英文）

The wave-induced vertical ship motions and bending moments of a double hull-oil tanker in realistic flooding conditions are studied. The scenarios investigated are represented by water ingress into the starboard ballast tanks for collision damage cases and both starboard and portside ballast tanks for grounding situations. Seakeeping computations are performed for eight damage scenarios and for the intact condition, each corresponding to different changes in displacement, trim, and heel. For each of the damage conditions, transfer functions of vertical motions and loads are calculated using a potential linear 3 D panel hydrodynamic code in the frequency domain that includes effect of the motion of the water in flooded tanks. A MATLAB code is developed to facilitate automated hydrodynamic simulation of many damage scenarios. Verification of seakeeping results is performed by comparing transfer functions with results of the previous study. Wave-induced vertical responses of damaged ship are then compared to those of intact ship using two spectral-based methods originating from uncertainty analysis of wave loads, which are convenient tools to assess consequences of damage on short-term ship responses. Generally, observed trend is that vertical wave-induced responses of damaged ship converge toward those of intact ship with increasing wave period. Fairly small differences between responses of asymmetrically damaged ship with respect to the symmetrical incoming wave directions are found. The results of the study are an efficient method for seakeeping assessment of damaged oil tankers and the framework for evaluating consequences of damage scenarios, heading angles, and sea conditions on seakeeping responses of damaged ships.The results can be used to decide if the intact ship model can be used instead of the damaged one for the emergency response procedure or for the risk assessment studies when modeling and computational time represent important limitations.     

水面舰船迎浪航行时大幅运动预报的切片算法

将预报船舶运动和波浪载荷的切片理论加以扩展，应用于有限深水中船舶迎浪航行时大幅纵向运动和波浪载荷的时域求解。预报结果体现出考虑了湿表面及化后引起的船舶运动和受力的非线性性质。本计算方法简便、实用，适用于船舶纵向大幅运动预报。     

Fourier NUBS method to express ship hull form

This article presents a method of numerical expression to draw ship hull forms. Recent developments in research into ship hull optimization need a method to express ship hull form as precisely as possible with a small number of design parameters. This method is based on a combination of the Fourier–sine series expansion and nonuniform B-spline (NUBS) interpolation. The merit of the combination is that it removes the wiggles which are often found when generating a rectangular-type curve, such as the midship section of tanker ship, with a simple Fourier series expansion. Here, the procedure is explained, and then a tanker ship hull is generated. Through the discussion, we show the effectiveness of the method.